The present invention relates to a small-sized electronic calculator.
Table-top electronic calculators and portable electronic calculators are generally well known. Some of them are connected to the household 110-volts AC power line, while many of them use suitable primary batteries, such as silver oxide, lithium or manganese cells. Battery-powered calculators are advantageous in that they can be used without any commercial electric power line, and they are ready for use at any desired place. Particularly, portable electronic calculators have which have the size of a usual credit card and use a thin silver oxide battery are well known. The life of such a conventional battery is relatively short because the user often forgets turning off the battery when the operation of the calculator is interrupted. It is troublesome for the user to exchange a used small battery with a new one. Further, since exhaustion of silver natural resources must be predicted, there is a strong demand for electronic calculators having no primary battery.
On the other hand, solar batteries are also well known. Single crystal silicon photocells have widely been developed and used. Single crystal silicon is most frequently used as the photovoltaic layer of solar batteries, because it is popular in the semi-conductor art and has a high efficiency of conversion of photo energy into electric energy per unit area.
Single crystal silicon is advantageous in the conversion efficiency as described above. However, a single crystal silicon chip is expensive, comparatively small in surface area, and readily broken down due to low mechanical (bending and impact) strength. To use a single crystal silicon photocell as the power source of an electronic calculator, a plurality of such photocells should be successively connected in series to one another to increase the output voltage. Electrical connection of the single crystal silicon chips raises the cost and the danger of fault or disconnection.
The demand for electronic calculators requiring neither exchange of a battery nor the charging of the battery is very strong. It is apparent that electronic calculators will be widely used in developing countries if those requiring no primary battery are made available. Strongly desired is an electronic calculator having as its electrical power source a solar battery which is inexpensive, easily manufactured and assembled, and reliable when compared with that having the single crystal silicon solar battery.